1. Field of the Invention
The present invention relates to a method of connection re-establishment and related communication device, and more particularly, to a method of connection re-establishment and related communication device for avoiding repeatedly triggering connection re-establishment procedure in a wireless communication system.
2. Description of the Prior Art
Long Term Evolution wireless communications system (LTE system) is an advanced high-speed wireless communications system established upon the 3G mobile telecommunications system, that supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B alone rather than in Node B and RNC (Radio Network Controller) respectively, meaning the system structure becomes simple.
In the LTE system, the Radio Resource Control (RRC) layer, a Layer 3 protocol, is the core of communications protocols related to access stratum (AS) and is located in evolved Node B (eNB) of the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and a user equipment (UE). The RRC layer uses RRC messages to perform RRC procedures. RRC messages are formed from many information elements (IEs) used for embedding necessary information for setting, changing, or releasing protocol entities of Layer 2 and Layer 1, thereby establishing, adjusting, or canceling information exchange channels to perform data packet transportation.
Generally speaking, when the UE has established the connection with the network, the connection may encounter some problems under certain circumstances. At this point, the UE attempts to recover the connection with the network through an RRC connection re-establishment procedure. Thus, when the connection between the UE and the network does not work functionally, the LTE system defines the following situations which can trigger an RRC connection re-establishment procedure.
(1) When radio link failure occurs, the UE re-establishes the connection with the network through the RRC connection re-establishment procedure.
(2) When handover failure occurs, the UE attempts to re-establish the connection with the network.
In addition, when the UE detects a physical layer radio link problem, the RRC layer starts a timer. If the physical layer radio link problem still exists when the timer expires, the RRC layer determines the radio link failure occurs. Generally, there are two other situations where the RRC layer determines the radio link failure occurs: (1) The MAC layer informs the RRC layer that random access problem occurs; (2) The RLC layer informs the RRC layer that a lower layer problem is detected. The lower layer problem is caused by the number of retransmission attempts of one RLC protocol data unit (PDU) exceeding a predefined value. As a result, the UE initiates the RRC connection re-establishment procedure.
Nevertheless, when the radio link problems are detected by the physical layer, the RRC layer starts the timer to perform radio link monitoring. In the meantime, the UE may not be able to successfully transmit RLC PDUs due to the radio link problem, causing the number of retransmission attempts to exceed the predefined value. Consequently, the RRC layer determines the radio link failure occurs and initiates the RRC connection re-establishment procedure. Once the timer expires, another RRC connection re-establishment procedure is triggered. As a result, the previous RRC connection re-establishment procedure may be aborted or an unexpected problem may occur.
To put it simply, the lower layer problem is detected prior to expiry of the timer, which triggers an RRC connection re-establishment procedure. Another RRC connection re-establishment is triggered when the timer expires. As a result, unexpected problem may occur, causing the connection malfunction.